In this work, reflection and transmission of shear waves from discontinuities in a plate waveguide are investigated using both analytical and finite element models. The study is motivated by the important role played by guided waves in nondestructive applications, ranging from detection of corrosion or cracks to the evaluation of the state of prestress [1]. In comparison to conventional ultrasonic methods, the advantage of guided waves is their larger inspection range, moreover, if compared to modal analysis they exhibit greater sensitivity because of the use of ultrasonic frequencies. Practical guided-wave ultrasonic testing is done by sending a signal along a waveguide and interpreting reflected and transmitted fields. In the presence of a discontinuity, this response consists of a complex superposition of waves. The characteristics of a discontinuity are related to the amplitude of scattered fields to obtain a quantitative evaluation of the discontinuity itself, and hence solve the inverse problem. The analytical model presented here makes use of the principle of reciprocity in elastodynamics [2, 3] to derive coefficients of reflection and transmission of a given incident wave from symmetric and asymmetric discontinuities [4]. The kinds of discontinuities considered are double sharp changes of cross-section, which conventionally represent notches. In parallel, finite element models are developed to corroborate analytical results. The change of reflection and transmission coefficients as a function of notch depth and extension is discussed for different values of the ratio wavelength/notch extension. It has been shown that, to detect small notches, a value of such ratio smaller or equal to 1 is required, meaning the need for high frequencies [5]. In this work it is shown that high frequencies are necessary not only to make the notch observable, but also to be able to observe its asymmetry, which requires that at least the cutoff frequency of the first asymmetric wave mode is exceeded. References [1] Pau, A. and Lanza di Scalea F., Nonlinear guided wave propagation in prestressed plates Journal of the Acoustical Society of America, 137 (3), page 1529-1540 (2015). [2] Auld, B.A., Acoustic Fields and Waves in Solids, Krieger, Malabar, FL (1999). [3] Achenbach, J.D., Reciprocity in Elastodynamics, Cambridge University Press, Cambridge, UK (2003). [4] Pau, A., Capecchi, D. and Vestroni, F., Reciprocity principle for scattered fields from discontinuities in waveguides, Ultrasonics, 55(1), page 85-91 (2015). [5] Pau, A. and Vestroni, F., Wave propagation in one-dimensional axial waveguides for damage characterization Journal of Intelligent Materials Systems and Structures, 22 (16), page 1869-1877 (2011).
Reflection and transmission of shear waves from discontinuities in a plate / Pau, Annamaria; Achillopulou, Dimitra. - (2015), pp. 116-116. (Intervento presentato al convegno XXII Congresso Associazione Italiana di Meccanica Teorica e Applicata tenutosi a Genova nel 14-17 settembre 2015).
Reflection and transmission of shear waves from discontinuities in a plate
PAU, Annamaria;
2015
Abstract
In this work, reflection and transmission of shear waves from discontinuities in a plate waveguide are investigated using both analytical and finite element models. The study is motivated by the important role played by guided waves in nondestructive applications, ranging from detection of corrosion or cracks to the evaluation of the state of prestress [1]. In comparison to conventional ultrasonic methods, the advantage of guided waves is their larger inspection range, moreover, if compared to modal analysis they exhibit greater sensitivity because of the use of ultrasonic frequencies. Practical guided-wave ultrasonic testing is done by sending a signal along a waveguide and interpreting reflected and transmitted fields. In the presence of a discontinuity, this response consists of a complex superposition of waves. The characteristics of a discontinuity are related to the amplitude of scattered fields to obtain a quantitative evaluation of the discontinuity itself, and hence solve the inverse problem. The analytical model presented here makes use of the principle of reciprocity in elastodynamics [2, 3] to derive coefficients of reflection and transmission of a given incident wave from symmetric and asymmetric discontinuities [4]. The kinds of discontinuities considered are double sharp changes of cross-section, which conventionally represent notches. In parallel, finite element models are developed to corroborate analytical results. The change of reflection and transmission coefficients as a function of notch depth and extension is discussed for different values of the ratio wavelength/notch extension. It has been shown that, to detect small notches, a value of such ratio smaller or equal to 1 is required, meaning the need for high frequencies [5]. In this work it is shown that high frequencies are necessary not only to make the notch observable, but also to be able to observe its asymmetry, which requires that at least the cutoff frequency of the first asymmetric wave mode is exceeded. References [1] Pau, A. and Lanza di Scalea F., Nonlinear guided wave propagation in prestressed plates Journal of the Acoustical Society of America, 137 (3), page 1529-1540 (2015). [2] Auld, B.A., Acoustic Fields and Waves in Solids, Krieger, Malabar, FL (1999). [3] Achenbach, J.D., Reciprocity in Elastodynamics, Cambridge University Press, Cambridge, UK (2003). [4] Pau, A., Capecchi, D. and Vestroni, F., Reciprocity principle for scattered fields from discontinuities in waveguides, Ultrasonics, 55(1), page 85-91 (2015). [5] Pau, A. and Vestroni, F., Wave propagation in one-dimensional axial waveguides for damage characterization Journal of Intelligent Materials Systems and Structures, 22 (16), page 1869-1877 (2011).I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
